//! Dynamic dispatch for layout implementations.
//! Each layout should at least implement these functions:
//! - handle(this: *@This(), event: Event) anyerror!*std.ArrayList(Event) {}
//! - render(this: *@This(), renderer: *Renderer) anyerror!void {}
//! - deinit(this: *@This()) void {}
//!
//! Create a `Layout` using `createFrom(object: anytype)` and use them through
//! the defined interface. The layout will take care of calling the correct
//! implementation of the corresponding underlying type.
//!
//! Each `Layout` is responsible for clearing the allocated memory of the used
//! widgets when deallocated. This means that `deinit()` will also deallocate
//! every used widget too.
//!
//! When `Layout.render` is called the provided `Renderer` type is expected
//! which handles how contents are rendered for a given layout.
const std = @import("std");
const isTaggedUnion = @import("event.zig").isTaggedUnion;

pub fn Layout(comptime Event: type, comptime Renderer: type) type {
    if (!isTaggedUnion(Event)) {
        @compileError("Provided user event `Event` for `Layout(comptime Event: type)` is not of type `union(enum)`.");
    }
    const Events = std.ArrayList(Event);
    const Type = struct {
        const LayoutType = @This();
        const Element = union(enum) {
            layout: LayoutType,
            widget: @import("widget.zig").Widget(Event, Renderer),
        };
        const Ptr = usize;
        pub const Interface = @import("interface").Interface(.{
            .handle = fn (anytype, Event) anyerror!*Events,
            .render = fn (anytype, *Renderer) anyerror!void,
            .deinit = fn (anytype) void,
        }, .{});

        const VTable = struct {
            handle: *const fn (this: *LayoutType, event: Event) anyerror!*Events,
            render: *const fn (this: *LayoutType, renderer: *Renderer) anyerror!void,
            deinit: *const fn (this: *LayoutType) void,
        };

        object: Ptr = undefined,
        vtable: *const VTable = undefined,

        // Handle the provided `Event` for this `Layout`.
        pub fn handle(this: *LayoutType, event: Event) !*Events {
            return try this.vtable.handle(this, event);
        }

        // Render this `Layout` completely. This will render contained sub-elements too.
        pub fn render(this: *LayoutType, renderer: *Renderer) !void {
            return try this.vtable.render(this, renderer);
        }

        pub fn deinit(this: *LayoutType) void {
            this.vtable.deinit(this);
            this.* = undefined;
        }

        pub fn createFrom(object: anytype) LayoutType {
            return LayoutType{
                .object = @intFromPtr(object),
                .vtable = &.{
                    .handle = struct {
                        // Handle the provided `Event` for this `Layout`.
                        fn handle(this: *LayoutType, event: Event) !*Events {
                            const layout: @TypeOf(object) = @ptrFromInt(this.object);
                            return try layout.handle(event);
                        }
                    }.handle,
                    .render = struct {
                        // Render the contents of this `Layout`.
                        fn render(this: *LayoutType, renderer: *Renderer) !void {
                            const layout: @TypeOf(object) = @ptrFromInt(this.object);
                            try layout.render(renderer);
                        }
                    }.render,
                    .deinit = struct {
                        fn deinit(this: *LayoutType) void {
                            const layout: @TypeOf(object) = @ptrFromInt(this.object);
                            layout.deinit();
                        }
                    }.deinit,
                },
            };
        }

        // import and export of `Layout` implementations
        pub const HStack = @import("layout/HStack.zig").Layout(Event, Element, Renderer);
        pub const VStack = @import("layout/VStack.zig").Layout(Event, Element, Renderer);
        pub const Padding = @import("layout/Padding.zig").Layout(Event, Element, Renderer);
        pub const Framing = @import("layout/Framing.zig").Layout(Event, Element, Renderer);
    };
    // test widget implementation satisfies the interface
    comptime Type.Interface.satisfiedBy(Type);
    // TODO: there is a dependency loop (due to the necessary `Layout` type for the `Element`)
    comptime Type.Interface.satisfiedBy(Type.HStack);
    comptime Type.Interface.satisfiedBy(Type.VStack);
    comptime Type.Interface.satisfiedBy(Type.Padding);
    comptime Type.Interface.satisfiedBy(Type.Framing);
    return Type;
}